Heterostructured Ni/NiO composite as a robust catalyst for the hydrogenation of levulinic acid to γ-valerolactone

A non-precious metal catalyst Ni/NiO is developed for the efficient hydrogenation of levulinic acid to γ-valerolactone under mild conditions. Treating nickel oxide in hydrogen at controlled temperature of 473–573 K results in its partial reduction to metallic nickel and the formation of Ni/NiO heterojunctions, as indicated by the characterization results from in situ XRD, XPS and TEM. The as-prepared Ni/NiO catalyst exhibits remarkable activity in levulinic acid hydrogenation with a high mass activity of 14.1 mmol/h/g at 393 K, being over 18 times higher than NiO and 10 times higher than metallic Ni. Besides, Ni/NiO shows very good stability and recyclability during the reaction, making it a promising catalyst for practical levulinic acid hydrogenation. The formation of Ni/NiO heterojunctions is crucial for the remarkable activity of Ni/NiO composite catalyst and a cooperative Langmuir-Hinshelwood mechanism is proposed for levulinic acid hydrogenation on the basis of kinetic analysis and theoretical calculations. The concept of cooperative catalysis on metal/oxide heterojunctions can be expanded to other hydrogenation reactions.